Start A Project

How to Cost Your Extension

Extending a home is a great way to add extra space to your home without having to move but trying to work out how much it’s going to cost can feel a bit overwhelming at first – especially trying to understand which rules and regulations will apply to your extension (all with their own costs).

If you’re not really interested in understanding the breakdown of costs and would rather just speak to someone about your project, you’re always welcome to book a free 20 minute call with one of our home improvement Architects about any questions you have.

Otherwise, let’s take a look at some numbers to give you a rough idea of how much it will cost to build your extension.


Build Costs

The main governing factor of the cost of an extension is square footage or square meterage – how big it is. So, the bigger it is, the more it’s going to cost. Typically if you worked out a square meter at £1500, that’s a good base price for basic quality; £1900 per square meter for good quality and £2200 per square meter for excellent quality.


Rear/Side Extension (Single Storey)

For an average single storey extension of 20 square metres, the build cost would be from £30,000 for basic quality, from £38,000 for good quality and from £44,000 for excellent quality.


Double Rear/Side Extension (Two Storey)

Budget from £53,000


Loft Conversion

(Best roof type for the loft extension is a gabled roof (2 sides slope) rather than a hipped roof (all 4 sides slope))

Types of Loft Conversions

  • Room Loft Conversion
    Budget from £15,000
  • Dormer Loft Extension
    Budget from £20,000 or from £35,000 with the inclusion of an en suite
  • ‘Raising the Roof’ or Changing the Roof Structure
    Budget from £40,000
  • Hip to Gable Roof Extension
    Budget from £25,000

A single storey extension of 20 square metres of basic quality will will cost you a minimum of £44,000 including most fees, but excluding interior fit-out costs.

Interior Fit-Out Cost Guide

(These costs are in addition to the build cost of the extension)


  • Kitchen
    Budget from £7,000 to £15,000 – the biggest cost factor is the appliances – the better the appliances, the higher the cost

  • Bathroom
    Budget around £5,000 assuming you can connect to existing pipework

  • Utility Room
    Budget from £1,500 to £11,000 depending on size and quality of appliances

  • Flooring
    Budget from £25 to £100 per square metre

  • Underfloor Heating
    Budget from £10,000 to £15,000

  • New Boiler
    Budget from £2,000 to £3,000 depending if the boiler is being fitted in an existing or new location

  • Wall and Ceiling Finishes
    Budget around £85 per square metre for plaster or dry-lining plus paint

  • Bi-fold or Sliding Doors
    Budget from £1,500 to £2,000 per linear metre

Professional Fees

  • Measured survey – £500 to £1,500

  • Planning and construction drawings – £2,700 to £4,000

  • Architect design fees – three to seven percent of the build cost (e.g. a fee of £900 to £2,100 for a £30,000 extension)

  • Structural engineer – £500 to £1,000

  • Contract management (employing someone to hire the builders/tradesmen) – three to seven percent of the build cost (e.g. a fee of £900 to £2,100 for a £30,000 extension)

  • Project management by a building contractor or architect will be 15 to 20 percent (up to £4,500) on top of the net cost of labour, materials and overheads

  • VAT at 20 percent of the labour, materials and services if applicable


An extension of 20 square metres at a build cost of £30,000 could potentially attract professional fees from £12,000.

FREE GUIDE: Category: Home Extensions
FREE: Home Extension Cost Guide

Inside this easy to ready free guide, you will learn how much extensions, loft conversions and other renovations to your home are typically priced.

Permission Fees

Additional reports that may be required

  • Tree reportfrom £250
  • Flood risk assessment – from £250
  • Ecology report  – from £400
  • Historic building report (if your home is a listed building)


  • Building Regulations Approval
    You don’t need to get approval yourself if you use someone registered with a competent person scheme. If you are carrying out the building works yourself, or you’re working with someone who’s not registered with a competent person scheme, then budget from £400 – £750

  • Party Wall Agreement
    (Covers shared walls/structures and protects both yourself and your neighbours whilst work is being carried out) You can issue a Party Wall Notice yourself (for no charge) to each of your affected neighbours. If your neighbour agrees to the works in writing, you will not require a Party Wall Agreement.

    If the neighbour refuses to give consent or fails to respond within 14 days, then a surveyor will need to be appointed to arrange a Party Wall Award that will set out the details of the work which will carry a fee of £700 to £1,000 per neighbour

  • Build Over Agreement
    (If you’ve got a shared drain running across the back of your garden and you’re going to build over it or within three meters proximity)
    Budget around £335


All together, it’s worth setting aside at least £2,000 to cover any fees for permissions and sign off



If you are carrying out conversion/extension works you should arrange specialist insurance to cover the new works and the existing structure as most home insurers won’t cover works if contracts exceed £50,000. For expert advise on this and any other insurance exposures, please contact Gary Philip at GPS Insurance Brokers on 020 8207 7385. GPS Insurance

Share this...

Share on facebook
Share on google
Share on twitter
Share on linkedin

Total Cost

Single Storey

For a single storey extension of 20 square metres of basic quality, this will give you a minimum spend of £44,000 including most fees, but excluding interior fit-out costs.


Two Storey

For a two storey extension, if the second storey is the same size as the first then the generally-accepted rule is to add 50% extra to the cost of a single storey extension plus an additional 10% for professional fees and VAT.

This gives you a minimum spend of about £87,000 including most fees, but excluding interior fit-out costs.


Getting An Accurate Figure

To calculate the cost of your project, Yoop Architects take a measured survey of your property, which captures all the information. Then we draw it all up in 3D and create sketches to show sometimes five or six options if it’s not entirely clear how you want to achieve your brief.

A lot of the times there are different ways of achieving your desired result and the more you work with the building, the cheaper, easier and faster your build will be. Once you’ve got a favorite scheme or a couple of favorite schemes, we have a phase called ‘Meet the Builders’. That’s when we can introduce you to a couple of builders, they visit the property and they give you a ballpark figure.


Thinking of your dream home improvement or how to start your project? 
Need your planning questions answered? Talk to an expert today.
Book a FREE 20 Minute Call with one of our home improvement Architects.

Connect with Yoop Architects...

Share on facebook
Share on google
Share on twitter
Share on linkedin

Post Author: yoopadmin